The present invention relates generally to the field of continuous flow gas humidification systems, and, more particularly, to a method and a system for humidifying gas reactants for fuel cells or other gases.
As described in U.S. Pat. No. 6,338,472 to Shimazu et al., humidifiers, like those of the field of the present invention, are typically used to humidify process gases supplied to an anode or a cathode of a solid polymer fuel cell. The process gases comprise a fuel gas provided to the anode and an oxidizing gas provided to the cathode. A solid polymer fuel cell generates electrical energy by electrochemical reactions in which protons generated from a fuel supplied to the anode transfer to the cathode through an electrolyte membrane and react with an oxidizing gas supplied to the cathode to produce water. The humidifier of the present invention, however, is not limited to fuel cells, but is generally applicable to the humidification of gases.
To operate a solid polymer fuel cell normally, the electrolyte membrane must be kept wet. To keep the electrolyte membrane wet, the process gases are typically humidified by one or more of a variety of techniques. For example, one commonly used technique, referred to herein as a xe2x80x9cbubbling-typexe2x80x9d humidifier, involves bubbling reactant gases up through a container of heated water so that water molecules are taken up with the reactant gases. An energy source is provided to facilitate the water evaporation into the gas bubbles or gas stream through the container. This method of humidification has the advantage of being very simple and inexpensive.
However, the degree of humidification depends on the height of the water column through which the gas bubbles travel, the size of the gas bubbles (and therefore the surface area of the bubbles in contact with the water), and the temperature of the water. Further, the evaporation of water under the dew point temperature is commonly relatively slow. When the flow rate of the gases through the system is slow, the total surface area of the gas bubbles is large, and evaporation of water into the gas approaches saturation. But, when flow rate is high, the size of the bubbles may be large, and there is therefore insufficient time for adequate evaporation and the humidification is commensurately below saturation.
Another known technique for humidifying reactant gases uses a xe2x80x9cmembrane-typexe2x80x9d humidifier. One example of a membrane-type humidifier is shown and described in U.S. Pat. No. 5,996,976 to Murphy et al. In this technique, water is pumped through a heating element and then directed to one side of a porous membrane. The gases to be humidified are directed across the other side of the membrane. Water molecules penetrate the membrane from the heated water side to the reactant gas side where the water molecules evaporate into the gases and the gases absorb heat from the water. The water may be circulated through a heating chamber as described, or the water may be heated directly in an evaporation chamber. The temperature of the gas-vapor mixture is lower than the temperature of the water because evaporation occurs at the surface of the membrane. Because of this phenomenon, the temperature and humidity of the gas-vapor mixture is rather difficult to control. Further, the difficulty of control increases as the rate of gas flow increases because the amount of heat absorbed from the water is relatively high. Further, a specialized membrane is required, increasing the overall cost of such a system.
Yet another technique for the humidification of a gas involves the application of ultrasonic energy to the gas and a water bath. A quantity of water is contained within an enclosure and gas is introduced to the volume within the enclosure above the surface of the water. An ultrasonic energy source within the enclosure extends through the gas volume into the water bath. Application of ultrasonic energy generates water vapor, which is taken up by the gas and the gas-vapor mixture is withdrawn from the enclosure. This technique has the advantage of easily controllable humidity of the gas-vapor mixture for xe2x80x9cbatchxe2x80x9d processing of gas, but is not suitable to generate and control the humidity of a continuous stream of gas.
Still another technique for humidification of a gas may be referred to as a xe2x80x9csteam-injection-typexe2x80x9d humidifier. In this technique, water is injected onto a hot element, such as a plate, to evaporate the water into an enclosure. Gas is pumped into the enclosure to mix with the water vapor to develop a gas-vapor mixture. The amount of water that is injected onto the heating element is calculated and controlled to meet certain humidity requirements. Further, the temperature of the exit gas-vapor mixture is controlled by controlling the temperature of the heating element.
However, this factor presents a drawback of this technique in that the heating component must use a certain minimum power to reach a temperature sufficiently high to flash the water to vapor instantly and this minimum temperature is usually much higher than the preferred mixture temperature. Also, it is difficult to quickly change the temperature of the heating element when the flow rate of gas or water changes and it is difficult to precisely control the temperature of the gas-vapor mixture, thus the mixture is likely to be overheated. Even if the mixture temperature can be adequately controlled, the range of flow rate and the range of temperature is unacceptably limited using this technique. This is because this technique requires the simultaneous control of two parameters, i.e. the temperature of the gas-vapor mixture and the temperature of the heating element, in one control loop by one means, i.e. the power to the heating element.
One proposed solution to this control problem involves the use of a condenser in the stream for the gas-vapor mixture. In principle, the humidification is carried out in two steps and two devices. The first step involves steam injection as previously described to generate an over-heated, over-humidified gas-vapor mixture. The second step involves passing the mixture through the condenser to condense the gas-vapor mixture at its dew point. A chiller is required to carry away the heat released from the condensation to maintain the condenser at the dew point. Thus, additional energy is needed to generate the over-heated and over-humidified mixture in the first step, and even more energy is required to drive the chiller to dissipate the additional heat from the cooling and condensation of the mixture. This means that this technique is very energy inefficient, and it is also bulky, complicated, and expensive to build and use.
Thus, there remains a need for a system and a method of humidifying gases that is energy efficient, simply, and easy to control, and that further provides a desired amount of humidification of a continuous gas stream. The present invention is directed to such a solution.
The present invention addresses these and other drawbacks in the art by providing a continuous flow of gas through two stages of bubbling with an intermediate heating stage. No permeable membrane is used, and only a single parameter is used for precise control of the humidification of the gas at the dew point. That is, the temperature of the bulk water is used as the parameter to control the operation of the heating element in the intermediate heating stage.
Gas is introduced into an enclosure by pumping the gas into a gas entry pipe or tube. The gas exits the gas entry pipe to bubble up through a bubbling evaporator. At the water surface of the bubbling evaporator, a heating element heats the gas-vapor mixture. The heated gas-vapor mixture then bubbles up through the bulk water, and the temperature of the bulk water determines the thermal cycles or activation of the heating element. The gas-vapor mixture bubbles up through the bulk water, reaching saturation at the dew point. The saturated gas is then directed from the humidifier for use.
It is therefore an object of the invention to provide a gas humidifier that is simple and easy to control to precisely produce saturated gas at the dew point. It is a further object of the invention to provide a method of humidifying gas with such a humidifier. It is yet another object of the invention to provide a gas humidifier that produces saturated gas at the dew point over a wide range of temperatures using a single measured parameter within the enclosure.
These and other features and advantages of this invention will be readily apparent to those skilled in the art.